Two-dimensional time delay integration visible cmos image sensor

ABSTRACT

A two dimensional time delay integration CMOS image sensor having a plurality of pinned photodiodes, each pinned photodiode collects a charge when light strikes the pinned photodiode, a plurality of electrodes separating the plurality of pinned photodiodes, the plurality of electrodes are configured for two dimensional charge transport between two adjacent pinned photodiodes, and a plurality of readout nodes connected to the plurality of pinned photodiodes via address lines.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

This application is a continuation of and claims the benefit and priority of U.S. application Ser. No. 11/683,811, entitled “TWO-DIMENSIONAL TIME DELAY INTEGRATION VISIBLE CMOS IMAGE SENSOR,” filed on Aug. 3, 2007, which is assigned to the assignee hereof and hereby expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a Complementary Metal Oxide Semiconductor (CMOS) image sensor. More particularly, the invention relates to two-dimensional time delay integration visible CMOS image sensor.

2. Description of Related Art

Unmanned Aerial Vehicles (UAVs) are remotely piloted or self-piloted aircrafts that can carry cameras, sensors, and other communication equipment. UAVs may be remotely controlled (e.g. flown by a pilot at a ground control station) or fly autonomously based on pre-programmed flight plans or more complex dynamic automation systems. UAVs are typically used for reconnaissance and intelligence-gathering, and for more challenging roles, including combat missions.

Ideally, an image taken from a camera onboard the UAV should be clear to provide accurate intelligence-gathering and determine appropriate targets. However, since UAVs shake from wind gusts during their flight operation, the image received from UAV is not clear enough to accurately identify targets on the ground. Consequently, there is a low signal to noise ratio due to wind and mechanical vibrations of the camera. This problem is compounded with moving scene imagery.

To improve signal to noise ratio, prior art stabilizers were integrated with the gimbal assembly of high speed cameras onboard the UAVs. The stabilizers reduce interferences caused by wind or mechanical vibrations. Additionally, the signal to noise ratio may be improved using Charge-Coupled Devices (CCDs) with Time Delay Integration (TDI). CCDs with TDI technology allow an image in a charge domain to move at about the same speed as the moving scene or target. However, CCDs with TDI are one dimensional and require multiple chip systems.

Conventional CMOS integrated circuits can achieve TDI in one dimension. The CMOS integrated circuits provide TDI using a switch matrix or a transistor chain CCD equivalent. The switch matrix typically accumulates additional noise and the signal to noise ratio improvement is less than proportional to the square root of the number of TDI channels. The transistor chain CCD equivalent cannot have high QE photodiode and is not a mainstream CMOS or CMOS Image Sensor (CIS) process.

With an ever increasing demand for improved imaging sensors, there remains a need for a two dimensional TDI visible CMOS image sensor that allow a charge to move at the same speed and follow a similar path in the charge domain as the moving image so that more charge from the scene can be integrated resulting in an improved signal to noise ratio. If readout noise is dominant, the signal to noise ratio improvement is proportional to the number of TDI channels.

SUMMARY OF THE INVENTION

The present invention fills this need by providing a time delay integration CMOS image sensor having a first pinned photodiode and a second pinned photodiode, the first pinned photodiode collects a charge when light strikes the first pinned photodiode, the second pinned photodiode receives the charge from the first pinned photodiode, and a plurality of electrodes in series located between the first and the second pinned photodiodes, the plurality of electrodes are configured to transfer the charge from the first pinned photodiode to the second pinned photodiode. The plurality of electrodes may be activated consecutively at different cycles.

In one embodiment, the time delay integration CMOS image sensor may include a plurality of readout nodes coupled to the second pinned photodiode via address lines. The number of readout nodes may be equal to the number of pinned photodiodes. The plurality of electrodes, the plurality of readout nodes and the address lines may form an orthogonal or hexagonal grid around the perimeter of each pinned photodiode.

BRIEF DESCRIPTION OF THE DRAWINGS

The exact nature of this invention, as well as the objects and advantages thereof, will become readily apparent from consideration of the following specification in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 is a prior art pinned photodiode with transfer gate and floating diffusion.

FIG. 2 illustrates the charge transport from the prior art pinned photodiode to the floating diffusion.

FIG. 3 is a timing diagram of the logic level for the transfer gate in FIG. 2.

FIGS. 4-8 illustrate charge transport in a CMOS image sensor, according to an embodiment of the invention.

FIG. 9 is a timing diagram of the logic level for the first electrode in FIGS. 4-8.

FIG. 10 is a timing diagram of the logic level for the second electrode in FIGS. 4-8.

FIG. 11 is a two dimensional time delay integration visible CMOS image sensor, according to an embodiment of the invention.

FIG. 12 is a two dimensional time delay integration visible CMOS image sensor, according to an embodiment of the invention.

FIG. 13 illustrates lateral charge transport in a two dimensional time delay integration visible CMOS image sensor, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Photodiodes are widely used in digital imaging devices to convert optical signals into electrical signals. Photodiodes may be arranged in linear or planar arrays with a plurality of photosensitive sensors, generally designated as pixels, on a semiconductor chip. Each pixel generates an output signal representing the amount of light incident on the pixel.

A pinned photodiode (PPD) is used to produce and integrate photoelectric charges generated in CCD or CMOS image sensors. FIG. 1 is a prior art pinned photodiode 11 with transfer gate 13 and floating diffusion 15. The pinned photodiode 11 generates a charge 17 while maintaining a fixed or pinned Fermi level 19. Regardless of the potential next to the Fermi level 19 of the pinned photodiode 11, the Fermi level 19 of the pinned photodiode 11 does not change.

Using the pinned photodiode 11 with transfer gate 13 allows for complete charge removal from light sensing area to the floating diffusion 15. FIG. 2 illustrates the charge transport from the pinned photodiode 11 to the floating diffusion 15. FIG. 3 is a corresponding timing diagram of the logic level for the transfer gate 13. In the first state 21, the charge 17 is collected by the pinned photodiode 11. The voltage on transfer gate 13 is zero in the first state 21. Next, in the second state 23, a positive voltage is applied to the transfer gate 13. This applied voltage attracts the charge 17 to move underneath the transfer gate 13, as shown in FIG. 2. Since the applied voltage decreases the quasi-Fermi level 19 underneath the transfer gate 13, charge 17 cannot move back to the pinned photodiode 11. In the third state 25, the applied voltage on transfer gate 13 is set to zero. Since the floating diffusion 15 has a quasi-Fermi level 19 that is lower than the Fermi level 19 of pinned photodiode 11, the charge 17 will move across to the floating diffusion 15.

Combining two transfer gates or electrodes in series provides charge transport from one pixel to the next. FIGS. 4-8 illustrate charge transport in a CMOS image sensor 27, according to an embodiment of the invention. The CMOS image sensor 27 has two or more electrodes 29 between pinned photodiodes 31. By using two or more electrodes 29, charge 36 can be moved from one pinned photo photodiode 31 to another. Preferably, the charge 36 moves at about the same speed as a moving image scene.

FIG. 4 shows a first electrode 32 and a second electrode 34 between pinned photodiodes 31. The CMOS image sensor 27 may have a plurality of pinned photodiodes 31 with electrodes 32 and 34 in between. Control logic may be used to operate the first electrode(s) 32 simultaneously. Control logic may also be used to operate the second electrode(s) 34 simultaneously and consecutive to the operation of the first electrode(s) 32. FIGS. 9 and 10 is an exemplary timing diagram of the logic level for the first electrode(s) 32 and second electrode(s) 34, respectively.

In operation, the CMOS image sensor 27 allows charge(s) 36 to travel from one pinned photo photodiode 31 to another. Initially, in FIG. 4, pinned photodiode 31 collects charge(s) 36 while maintaining a fixed or pinned Fermi level 38. Regardless of the potential next to the Fermi level 38 of the pinned photodiode 31, the Fermi level 38 of the pinned photodiode 31 does not change. No voltage is applied to the first and second electrodes 32 and 34.

Next, in FIG. 5, the first electrode 32 is activated by applying a voltage for a predetermined period. This voltage attracts the charge 36 to move underneath the first electrode 32. Since the applied voltage decreases the quasi-Fermi level 38 of the first electrode 32 by creating a well, charge 36 cannot move back to the pinned photodiode 31.

In FIG. 6, the second electrode 34 is activated by applying a positive voltage for a predetermined period. The voltage applied to the second electrode 34 is preferably greater than or equal to the voltage applied to the first electrode 32. The voltage applied to the second electrode 34 attracts the charge 36 to move underneath the second electrode 34 as well. The applied voltage decreases the quasi-Fermi level 38 of the second electrode 34 to allow charge 36 to distribute under both electrodes 32 and 34.

In FIG. 7, the applied voltage for the first electrode 32 is set to zero. This resets the potential of the first electrode 32 and collapses the well underneath the first electrode 32. Since the second electrode 34 is still activated, the quasi-Fermi level 38 of the second electrode 34 will be lower than the quasi-Fermi level 38 underneath first electrode 32 and photodiode 31. Consequently, the charge 36 that was underneath the first electrode 32 will move across and remain underneath the second electrode 34.

In FIG. 8, the applied voltage for the second electrode 34 is set to zero. This resets the potential of the second electrode 34 and collapses the well underneath the second electrode 34. Since the pinned photodiode 31 has a predetermined fixed Fermi level 31, the charge 36 underneath the second electrode 34 will move across to the adjacent pinned photodiode 31. Consequently, lateral charge 36 transport occurs in the CMOS image sensor 27.

According to an embodiment of the invention, the lateral charge 36 transport occurs over a 4 cycle period, as shown in FIGS. 9 and 10. A person skilled in the art would appreciate that different cycles may be used without departing from the spirit of the invention. In the first cycle, the first electrode 32 is activated by applying a positive voltage. In the second cycle, the second electrode 34 is activated as well by applying a voltage. In the third cycle, the first electrode 32 is deactivated by setting the voltage applied to the first electrode 32 to zero. In the fourth cycle, the second electrode 34 is deactivated by setting the voltage applied to the second electrode 34 to zero.

As shown in FIGS. 4-8, the CMOS image sensor 27 has a plurality of pinned photodiodes 31 with at least two electrodes 32 and 34 in between. Electrodes 32 operate at a different phase than electrodes 34 to allow charge 36 to move from underneath one electrode to the other. The phase relationship between electrodes 32 and electrodes 34 defines the transport direction of the charge 36. For example, control logic may be used to alternate the phase shift between the electrodes 34 and 34 such that the charge 36 moves from photodiode 31 adjacent to the second electrode 34, to underneath second electrode 34, to underneath first electrode 32, and finally to the photodiode 31 adjacent to the first electrode 32.

FIG. 11 is a two dimensional time delay integration visible CMOS image sensor 40, according to an embodiment of the invention. The sensor 40 has an active array of pixels 42, each pixel 42 may include a pinned photodiode 44 with four orthogonal electrodes 46, 47, 48 and 49. The pixels 42 are interconnected in a grid with readout nodes 50 and address lines 52. The address lines 52 control the voltage on electrodes 46, 47, 48 and 49. Through an additional transfer gate (not shown) between photodiode 44 and readout node 50, the signal charge can be transferred to the readout node 50 at the end of a TDI cycle. In one embodiment, the sensor 40 has a readout node 50 for every photodiode 44.

With moving scene imagery, pinned photodiode 44 of the time delay integration visible CMOS image sensor 40 generates a charge that moves in two dimensions at about the same speed and follows a similar path as the moving image. Similarly, mechanical vibrations of a camera cause random walk of any image point on the sensor 40. FIG. 11 illustrates the two dimensional charge transport directions 54 and 56. The charge moves laterally from one photodiode 44 to another. This lateral movement of charge provides improved charge integration from the moving scene. Since there are multiple readout nodes 50 distributed evenly in the sensor 40, photo-generated signals may be read at any point in the array closest to the readout node 50, rather than transporting the charge for readout down or up stream. This provides high frame rate capability with improved signal to noise ratio for the sensor 40.

To better approximate the curved random walk of a scene, the sensor may be configured to allow for charge transport in three or more directions. FIG. 12 illustrates charge transport in three directions 62, 64 and 66 for a two dimensional time delay integration visible CMOS image sensor 60, according to an embodiment of the invention. The sensor 60 has an active array of pixels 68, each pixel 68 may include a pinned photodiode 70 with six electrodes 72, 74, 76, 78, 80 and 82. The pixels 68 are interconnected in a polygonal grid, such as a hexagonal grid, with readout nodes 84 and address lines 86. The address lines 86 control the voltage on the electrodes 72, 74, 76, 78, 80 and 82. Through an additional transfer gate (not shown) between photodiode 70 and readout node 84, the signal charge can be transferred to the readout node 70 at the end of a TDI cycle. In one embodiment, the sensor 60 has a readout node 84 for every photodiode 70.

With moving scene imagery, pinned photodiode 70 of the time delay integration visible CMOS image sensor 60 generates a charge that moves in two dimensions at about the same speed and follows a similar path as the moving image. Similarly, mechanical vibrations of a camera cause random walk of any image point on the sensor 40. FIG. 13 illustrates lateral charge transport in sensor 60. Due to the hexagonal grid configuration, the charge travels in a smooth path 88 that follows the moving image. This lateral movement of charge provides improved charge integration from the moving scene. Since there are multiple readout nodes 84 distributed evenly in the sensor 60, photo-generated signals may be read at any point in the array closest to the readout node 84. This provides high frame rate capability with improved signal to noise ratio for the sensor 60.

A person skilled in the art would appreciate the potential applications of the two dimensional time delay integration visible CMOS image sensor of the present invention. The sensor may be used for translational image stabilization during single frame integration time. For example, very high bandwidth of translational vibrations can be stabilized from about 30 Hz to about 1 MHz. The maximum translational vibration amplitude may be limited by imager resolution. The sensor may also be used for rotational image stabilization during single frame integration time. For example, very high bandwidth of rotational movement can be stabilized from about 30 Hz to about 1 MHz. The maximum rotational vibration amplitude may be limited by pixel size and tolerable distortions.

Other applications of the sensor include residue light photography without tripod or flash, TDI camera with increased alignment tolerance and flow cytometry for capturing images of moving cells in fluids. The sensor may also be used, in combination with a stabilized gimbal, to enhance pointing accuracy to a few tens of grads. Additionally, the sensor may be used with Inertial Measurement Unit (IMU) to suppress random motion. Depending on frame rate, IMU may be replaced with processing algorithm.

While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein. 

1. An image sensor comprising: first and second regions, the first region located adjacent to the second region; a first photodiode located adjacent to the first region; a second photodiode located adjacent to the second region; a first transfer gate positioned above the first region, and configured to transfer a charge from the first photodiode to the first region; and a second transfer gate positioned above the second region, configured to receive the charge in the second region, and configured to transfer the charge from the second region to the second photodiode.
 2. The image sensor of claim 1, wherein the first and second regions are substantially free of any floating diffusion region or drain region.
 3. The image sensor of claim 1, further comprising: a first readout node located outside the first and second regions, and configured to collect a first charge from the first photodiode.
 4. The image sensor of claim 3, further comprising: a third transfer gate coupled between the first readout node and the first photodiode, and configured to facilitate the first node to collect the first charge from the first photodiode.
 5. The image sensor of claim 1, further comprising: a second readout node located outside the first and second regions, and configured to collect a second charge from the second photodiode.
 6. The image sensor of claim 5, further comprising: a fourth transfer gate coupled between the second readout node and the second photodiode, and configured to facilitate the second node to collect the second charge from the second photodiode.
 7. The image sensor of claim 1, wherein the first transfer gate is located directly adjacent to the second transfer gate.
 8. The image sensor of claim 1, wherein: the first photodiode has a first Fermi level, the second photodiode has a second Fermi level substantially the same as the first Fermi level, the first region has a first region Fermi level substantially higher than the first Fermi level, and the second region has a second region Fermi level substantially the same as the first region Fermi level.
 9. The image sensor of claim 8, wherein: the first transfer gate creates a first well in the first region, such that the first well has a first quasi-Fermi level substantially lower than the first Fermi level when the charge is transferred from the first photodiode to the first well, and the second transfer gate creates a second well in the first region, such that the second well has a second quasi-Fermi level substantially similar to the first quasi-Fermi level when the charge is transferred from the first well to the second well.
 10. The image sensor of claim 9, wherein: the first transfer gate restores the first region Fermi level in the first region after the charge is transferred from the first region to the second region, and the second transfer gate restores the second region Fermi level in the second region to transfer the charge from the second region to the second photodiode.
 11. A method for transferring a charge between a first photodiode and a second photodiode, comprising the steps of: creating a first well adjacent to the first photodiode during a first time period; and creating a second well adjacent to the first well and the second photodiode during a second time period partially but not entirely overlapping with the first time period.
 12. The method of claim 11, wherein the creating the first well step includes: adjusting, in a first region adjacent to the first photodiode, a first quasi-Fermi level substantially lower than a first Fermi level of the first photodiode.
 13. The method of claim 12, wherein the creating the second well step includes: adjusting, in a second region adjacent to the second photodiode and the first region, a second quasi-Fermi level substantially the same as the first quasi-Fermi level and substantially lower than a second Fermi level of the second photodiode.
 14. The method of claim 11, further comprising the steps of: collapsing the first well after the first time period; and collapsing the second well after the second time period.
 15. The method of claim 14, wherein: the collapsing the first well step includes restoring, in a first region adjacent to the first photodiode, a first region Fermi level substantially higher than a first Fermi level of the first photodiode, and the collapsing the second well step includes restoring, in a second region adjacent to the second photodiode and the first region, a second region Fermi level substantially higher than a second Fermi level of the second photodiode.
 16. The method of claim 11, wherein: the first time period has a first portion distinct from the second time period, and the second time period has a second portion distinct from the first time period.
 17. A method for transferring a charge between a first photodiode and a second photodiode, comprising the steps of: applying a first voltage to a first electrode to adjust a first quasi-Fermi level of a first region located adjacent to the first photodiode during first and second cycles; and applying a second voltage to a second electrode to adjust a second quasi-Fermi level of a second region located adjacent to the first region and the second photodiode electrode during the second cycle and a third cycle.
 18. The method of claim 17, wherein: the first quasi-Fermi level is substantially lower than a first Fermi level of the first photodiode, and the second quasi-Fermi level is substantially the same as the first quasi-Fermi level and substantially lower than a second Fermi level of the second photodiode.
 19. The method of claim 17, further comprising the steps of: floating the first electrode to restore a first region Fermi level of the first region after the second cycle; and floating the second electrode to restore a second region Fermi level of the second region after the third cycle.
 20. The method of claim 19, wherein: the first region Fermi level of the first region is substantially higher than the first Fermi level of the first photodiode, and the second region Fermi level of the second region is substantially higher than the second Fermi level of the second photodiode. 